At present, most matrix based display technologies are in its technological infancy compared to long established electronic image forming technologies such as Cathode Ray Tubes (CRT). As a result, many domains of image quality deficiency still exist and cause problems for the acceptance of these technologies in certain applications.
Matrix based or matrix addressed displays are composed of individual image forming elements, called pixels (Picture Elements), that can be driven (or addressed) individually by proper driving electronics. The driving signals can switch a pixel to a first state, the on-state (at which luminance is emitted, transmitted or reflected), to a second state, the off-state (at which no luminance is emitted, transmitted or reflected)—see for example EP-117335—or for some displays, one or any intermediate state between on or off (modulation of the amount of luminance emitted, transmitted or reflected)—see for example EP-0462619 and EP-117335.
Since matrix addressed displays are typically composed of many millions of pixels, very often pixels exist that are stuck in a certain state (on, off or anything in between). Where pixel elements comprise multiple sub pixels, individually controllable or not, then one or more of the sub-pixel elements may become stuck in a certain state. For example, a pixel structure may comprise three sub-pixel elements for red, green and blue colours respectively. If one of these sub-pixel elements becomes stuck in a certain state, then the pixel structure has a permanent colour shift. Mostly such problems are due to a malfunction in the driving electronics of the individual pixel (for instance a defect transistor). Other possible causes are problems with various production processes involved in the manufacturing of the displays, and/or by the physical construction of these displays, each of them being different depending on the type of technology of the electronic display under consideration. It is also possible that a pixel or sub-pixel element is not really stuck in a state, but shows a luminance or colour behaviour that is significantly different from the pixels or sub-pixels in its neighbourhood. For instance, but not limited to: a defective pixel shows a luminance behaviour that differs more than 20% (at one or more video levels) from the pixels in its neighbourhood, or a defective pixel shows a dynamic range (maximum luminance/minimum luminance) that differs more than 15% from the dynamic range of pixels in its neighbourhood, or a defective pixel shows a colour shift greater than a certain value comparing to an average or desired value for the display. Of course other rules are possible to determine whether a pixel or sub-pixel is defective or not (any condition that has a potential danger for image misinterpretation can be expressed in a rule to determine whether a pixel is a defective pixel). Bright or dark spots due to dust for example may also be considered as pixel defects. The exact reason for the defective pixel is not important for the present invention.
Defective pixels or sub-pixels are typically very visible for the user of the display. They result in a significantly lower (subjective) image quality, can be very annoying or disturbing for the display-user and for demanding applications (such as medical imaging, in particular mammography) the defective pixels or sub-pixels can even make the display unusable for the intended application, as it can also result in wrong interpretation of the image being displayed. For applications where image fidelity is required to be high, such as for example in medical applications, this situation is unacceptable.
U.S. Pat. No. 5,504,504 describes a method and display system for reducing the visual impact of defects present in an image display. The display includes an array of pixels, each non-defective pixel being selectively operable in response to input data by addressing facilities between an “on” state, whereat light is directed onto a viewing surface, and an “off” state, whereat light is not directed onto the viewing surface. Each defective pixel is immediately surrounded by a first ring of compensation pixels adjacent to the central defective pixel. The compensation pixels are immediately surrounded by a second ring of reference pixels spaced from the central defective pixel. The addressing circuit-determined value of at least one compensation pixel in the first ring surrounding the defective pixel is changed from its desired or intended value to a corrective value, in order to reduce the visual impact of the defect. In one embodiment, the value of the compensation pixels is selected such that the average visually defected value for all of the compensation pixels and the defective pixel is equal to the intended value of the defective pixel. In another embodiment, the values of the compensation pixels are adjusted by adding an offset to the desired value of each compensation pixel. The offset is chosen such that the sum of the offset values is equal to the intended value of the defective pixel.
It is a disadvantage of the solution proposed in the above document that a trial and error method is required for every other display in order to obtain a reasonable correction result.
From WO 03/100756 it is known to mask a faulty pixel having a defect sub-pixel for a display system with pixels having a set of primary sub-pixels with an additional redundant sub-pixel. The masking is performed by reducing an error between a desired perceptive characteristic of said faulty pixel and modified perceptive characteristics of said pixel. In other words, the method is focussed on obtaining a desired perceptive characteristic for the faulty pixel, whereby the use of a redundant sub-pixel is required. It is a disadvantage of the method of the above document that a redundant sub-pixel is necessary for each and every pixel. The document does not describe how to mask defects in a display system without additional redundant pixel.